System and method for determining fiber characteristics

ABSTRACT

A system is provided for characterizing optical fibers carrying signal traffic. The system includes a transmitter, a variable optical attenuator (VOA), a receiver, and a computing device. The transmitter propagates an optical test signal along a channel of a fiber pathway. The VOA adjusts the attenuation of the optical test signal from an initial, greater attenuation to a subsequent, lesser attenuation. At the same time, the computing device monitors at least one other channel of the fiber pathway and identifies effects upon the other channel(s) from the optical test signal. The computing device may communicate with the VOA and with other components of the fiber pathway to direct adjustment of the signal strength. A maximum optical test signal strength may thus be achieved that does not negatively affect signal traffic on the other channels, and the fiber pathway may subsequently be tested using the achieved maximum optical test signal strength.

RELATED APPLICATIONS AND PRIORITY CLAIM

This is a Continuation of prior application Ser. No. 11/690,390, filedMar. 23, 2007, now U.S. Pat. No. 7,474,830 entitled, “SYSTEM AND METHODFOR DETERMINING FIBER CHARACTERISTICS,” to which priority under 35U.S.C. §120 is claimed. The entire contents of the aforementionedapplication are herein expressly incorporated by reference.

BACKGROUND

As the pace of life accelerates, more people are demanding fast andreliable access to information, including audio and video content, fromtheir content service providers. As a result, content service providersmust find ways to update their systems to support the transmission oflarger volumes of content at higher transmission rates. The rate ofcontent transmission is typically described in terms of the bit rate ofthe connection or transfer. The bit rate is the number of bits that canbe conveyed or processed per unit of time (e.g., gigabits per second, orGb/s).

In fiber optic networks, where bits are conveyed via an optical signalthat is propagated along an optical fiber, increasing the bit rate ofcontent transmission is not as simple as pushing more bits per secondthrough the fiber. As the bit rate is increased, for example from 2.5Gb/s to 40 Gb/s, the sensitivity of the optical signal to attributes ofthe fiber is heightened. For example, deviations in the cross-section ofthe fiber and/or twisting of the fiber, which may have no significanteffect on optical signals propagated at 2.5 Gb/s, may result innoticeable signal distortions at 40 Gb/s. Fiber attributes may includethe optical signal-noise ratio (OSNR), chromatic dispersion (CD),polarization mode dispersion (PMD), pass band shape, and nonlineareffects. As a result, the fiber must be carefully characterized so thatthe fiber pathway can be tuned to accommodate the faster bit rate beforethe fiber system is put on-line,

Many fiber optic systems use Wavelength Division Multiplexing (WDM) tocombine multiple optical signals on a single fiber, using differentwavelengths of light to carry different signals. As an example, oneoptical fiber may have the capacity to carry eight signals on eightwavelengths, or channels. Most WDM systems are not fully populated,meaning that not all of the channels of a given fiber are carryingsignals at any given time. However, using one of the available channelsas a test channel for characterizing the fiber's attributes may causeinterference with the optical signals carried on the live channels ofthe same fiber, thereby negatively affecting the quality of thetransmission on those channels.

As a result, some have used the characterization of dark fibers, orfibers that carry no signal traffic on any channels, to estimate theproperties of other, lit fibers in the same cable. However, although adark fiber itself may be accurately characterized, the dark fiber mayhave properties that are slightly different than those of the otherfibers, even in the same cable. In addition, as the demand on fiberoptic systems increases, the number of dark fibers available for testingdecreases, making statistical estimates less reliable.

Thus, there is a need for a system and method for characterizing litoptical fibers, such as in a WDM system, without adversely affecting thetransmission performance of the fiber optic system.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic illustration of a fiber optic cable having twelveoptical fibers;

FIG. 2 is a schematic block diagram of a system according to oneembodiment showing a second variable optical attenuator;

FIG. 3 is a schematic block diagram of a system according to oneembodiment showing a pair of optical switches;

FIG. 4 is a schematic block diagram of a system according to oneembodiment showing a tunable dispersion compensator; and

FIG. 5 is a flow chart illustrating embodiments of a method forcharacterizing optical fibers carrying signal traffic according to oneembodiment.

DETAILED DESCRIPTION

Exemplary embodiments now will be described hereinafter with referenceto the accompanying drawings, in which exemplary embodiments andexamples are shown. Like numbers refer to like elements throughout.

Apparatuses and methods for characterizing optical fibers carryingsignal traffic are provided in accordance with various exemplaryembodiments. In general, systems and methods are described for adjustingthe attenuation of an optical test signal from a greater level ofattenuation, at which, for example, none of the optical test signal isallowed to propagate along a fiber pathway, to a subsequent, lesserlevel of attenuation, at which, for example, all of the optical testsignal is allowed to propagate along the optical fiber. By monitoringthe effect of the optical test signal on one or more other channels ofthe fiber pathway, it may be possible to achieve a maximum optical testsignal strength at which the other channels of the fiber pathway, whichmay be carrying signal traffic, remain unaffected. In this way, a livefiber pathway may be tested using the maximum optical test signalstrength without affecting signal traffic being propagated on otherchannels of the same optical fiber.

Referring to FIG. 1, individual optical fibers may be bundled to form afiber optic cable 10 to facilitate handling and installation. Forexample, twelve optical fibers may be bundled together, as in the cable10 of FIG. 1. A fiber optic cable 10 may include optical fibers that arecarrying signal traffic, sometimes called “lit” or “live” fibers 12, aswell as optical fibers carrying no signal traffic, called “dark” fibers14. In Wavelength Division Multiplexing (WDM) systems, multiple signalsare multiplexed onto a single fiber with each signal propagating on adifferent wavelength or channel. At any given time, certain channelswithin a particular fiber may be lit while others remain dark. Uponreaching a far end of a fiber, signals can be demultiplexed andrecovered as is well known.

The testing of optical fibers, including the measurement of certainfiber attributes and characteristics, generally requires the propagationof an optical test signal along the fiber pathway. The fiber pathway isthe path followed by an optical signal from one end to another and mayinclude one or more optical fibers, as well as various pieces ofequipment such as optical-to-electrical-to-optical translationequipment, multiplexers, optical amplifiers, optical add/drop modules,demultiplexers, and other equipment responsible for maintaining thequality of the signal as it is propagated. When an optical test signalis propagated along a dark channel of a lit optical fiber 12, however,signals being propagated along live channels within the fiber 12 may beadversely affected. For example, depending on the strength of theoptical test signal, the optical test signal may interfere with theother signals propagated on the same fiber, thereby degrading thequality of the other signals.

Nonetheless, it is often necessary to test lit optical fibers 12directly, as alternative testing methods may yield inaccurate results,may be too costly, and/or may disrupt the optical network. For example,testing a dark fiber 14, even a dark fiber in the same cable 10, may beinadequate as the dark fiber 14 may have different characteristics thanthe actual lit fiber 12. Although the differences may be slight, anyinaccurate characterization of the lit fibers 12 may be significant,especially when the lit fibers 12 are being tested in preparation forsignal transmission at increasingly higher bit rates, such as 40 Gb/s.In addition, the number of dark fibers 14 in each cable 10 iscontinually decreasing as more and more optical fibers are being used tocarry signal traffic. As a result, the statistical significance ofmeasurements of the dark fibers 14 is continually decreasing.

Alternatively, the characteristics of the optical fibers may beestimated, and the system may then be tuned to adjust for anyinaccuracies in the estimations. As previously mentioned, however, evenminor inaccuracies in the estimations may significantly affect systemperformance, especially at higher bit rates, and relying on systemtuning processes may be costly from an operations point of view as wellas a customer satisfaction point of view. Likewise, routing signaltraffic to so-called “protection routes,” which are intended as back-upfiber pathways, during testing of the previously lit fibers 12 may makethe fiber optic network vulnerable in case of an actual system failureevent or disruption as there would be no back-up systems available.Furthermore, characterizing the optical fibers during a maintenanceperiod may result in unacceptable delays to the testing, and the systemmay need to be shut down during the maintenance period presentingfurther obstacles to testing.

One of the problems associated with testing lit fibers 12 directly isthe risk that the optical test signal will interfere with signal trafficon other channels in the same fiber. Referring to FIG. 2, a system 5 isillustrated according to an exemplary embodiment including a firstvariable optical attenuator (VOA) 20 configured to adjust attenuation ofthe optical test signal from an initial, greater attenuation to asubsequent, lesser attenuation. At the same time, a computing device 22configured to communicate with the first VOA 20 and with other aspectsof the fiber pathway 24, as described below, is configured to monitor atleast one other channel of the fiber pathway 24 and to identify effectsupon the other channel(s) that are attributable to the optical testsignal. As will be explained below, a maximum optical test signalstrength may thus be achieved that does not negatively affect signaltraffic in the other channels, and the fiber pathway may subsequently betested using the achieved maximum optical test signal strength.

The system 5 according to one embodiment for characterizing opticalfibers carrying signal traffic includes a transmitter 26 at a first endof the fiber pathway, the first VOA 20, a receiver 28, and the computingdevice 22. The transmitter 26 is configured to generate or otherwiseprovide an optical test signal having a predefined wavelength forpropagation along a channel of the fiber pathway 24. The first VOA 20may be disposed proximate or at the first end of the fiber pathway 24,for example immediately downstream of the transmitter 26 or at leastbetween the transmitter and the first end of the fiber pathway. Thefirst VOA is configured to attenuate the strength of the optical testsignal propagated by the transmitter 26. The receiver 28 is configuredto receive the optical test signal that has been propagated along thefiber pathway 24 and, as such, may be located at any point downstream ofthe transmitter 26 and the first VOA 20. For example, the receiver 28may be located at the other, e.g., opposite, end of the fiber pathway24, such that a long stretch of the fiber pathway 24 (for example, thefiber pathway 24 extending between two cities) may be tested. Similarly,the receiver 28 may be located at a point along the fiber pathway 24such that only a portion of the fiber pathway 24, i.e., the portionbetween the transmitter 26 and the receiver 28 may be tested.

The computing device 22 is generally configured to communicate with thetransmitter 26, the first VOA 20, and the receiver 28. The computingdevice 22 may be a laptop computer, personal computer, server computer,workstation, or any computerized device capable of performing thefunctions described below in relation to the computing device. Aspreviously mentioned, the computing device 22 is configured to monitorat least one other channel and to identify effects upon the otherchannel(s). For example, as illustrated in FIG. 2, the computing device22 may be connected to a communications network 30, such as the Internetor other type of network, that supports the communication of databetween the computing device 22 and the various components of the system5. The connection 31 may be wireless, e.g., using radio frequency (RF),Bluetooth (BT), infrared (IrDA), or other wireless communicationtechniques, or the connection may be wired, e.g., using optical fiber orcopper wire to transmit data. In any case, the computing device 22 may,for example, receive data indicating a condition of the channel(s)carrying signal traffic from at least the receiver 28. Furthermore, thecomputing device 22 may direct the first VOA 20 regarding how to adjustthe attenuation of the optical test signal in response to the datareceived. The computing device 22 may also direct other components ofthe system 5, such as the transmitter 26 and the receiver 28.

For example, the computing device 22 may initially direct the first VOA20 to attenuate all or substantially all of the optical test signal tobe propagated along the fiber pathway 24. The computing device 22 maythen direct the transmitter 26 to generate or otherwise provide theoptical test signal for propagation along the optical fiber followingattenuation by the first VOA 20. The computing device 22 may direct thetransmitter 26, the first VOA 20, and/or other components automatically,e.g., according to instructions embodied in a software program executedby a processing element, or according to input provided by a user, e.g.,via a keyboard, mouse, or other input device. Once signals arepropagated along the optical fiber, the computing device monitors one ormore channels of the fiber pathway 24, that is, the channels other thanthat channel via which the optical test signal is propagated. As thecomputing device 22 continues to monitor the one or more channelscarrying signal traffic in the fiber being tested, the computing device22 may direct the first VOA 20 to gradually decrease attenuation of theoptical test signal. As long as the computing device 22 receives noindication that the signal traffic of the live channels is negativelyaffected, the computing device 22 may continue to direct the first VOA20 to decrease attenuation of the optical test signal until, forexample, the attenuation reaches zero or a value close to zero. In thecase where there is no attenuation or substantially no attenuation, theoptical test signal may be propagated along the fiber pathway 24 at amaximum signal strength (i.e., without any attenuation), and theappropriate tests for characterizing the fiber pathway 24 may beconducted at that maximum signal strength or any lesser strength, asrequired by the specific tests or measurements.

Similarly, the computing device 22 may be configured to direct the firstVOA 20 to stop adjusting attenuation of the optical test signal when apredefined optical test signal strength is achieved. For example,various tests for determining different characteristics of the fiber mayrequire different optical test signal strengths. However, once theoptical test signal strength corresponding to the highest strengthrequired is propagated along the fiber pathway 24 without any adverseeffects on the other channels, there is no further need to achievehigher signal strengths as all testing may be conducted using theachieved optical test signal strength. Thus, there would be no need tofurther decrease attenuation of the optical test signal.

The computing device 22 may be configured to direct the first VOA 20 tostop decreasing the attenuation of the optical test signal when theeffects upon the monitored channels result in a predefined alarmcondition. For example, the computing device 22 may be configured tomonitor the strength of the signal traffic on the live channels at oneor more points along the fiber pathway 24. A decrease in the strengthbelow a certain threshold level may result in an alarm condition, atwhich point the first VOA 20 would be stopped from further decreasingthe attenuation of the optical test signal to prevent additional adverseeffects on the live channels. The alarm condition may be defined inother manners. For example, the quality of the signal traffic on thelive channels may be monitored, such as by monitoring thesignal-to-noise ratio, and an indication of a signal quality that fallsbelow a certain threshold level may cause the computing device 22 todirect the first VOA 20 to cease decreasing the attenuation of theoptical test signal. Furthermore, in any of these examples, thecomputing device 22 may direct the VOA 20 to increase attenuation of theoptical test signal until the respective alarm condition is cleared. Inthis way, the maximum optical test signal strength at which no alarmconditions are caused may be achieved.

As another example, the computing device 22 may be configured to monitora bit error rate of the other channel(s) of the fiber pathway 24. Thecomputing device 22 may be further configured to direct the first VOA 20to adjust attenuation of the optical test signal to allow propagation ofthe test signal at a strength at which the bit error rate of the livechannels is below a predetermined level. For example, it may bedetermined that a bit error rate of 10⁻⁹ is the highest acceptable biterror rate. Thus, in this example, any bit error rate higher than 10⁻⁹would result in the computing device 22 directing the first VOA 20 tocease decreasing attenuation of the optical test signal. Also, asdescribed above with respect to alarm conditions, the computing device22 may direct the VOA 20 to increase attenuation of the optical testsignal until the bit error rate falls below a predetermined thresholdlevel. In this way, the maximum optical test signal strength at whichthe bit error rate is acceptable may be achieved. Regarding bit errorrates as well as alarm conditions, it is to be understood that themaximum achieved optical test signal strength may be less than therequired optical test signal strength for certain tests, and so it maynot be possible to perform all required tests on the lit fiber.

The computing device 22 may be capable of monitoring other parameters inaddition to those mentioned above to determine the effect of the opticaltest signal on the other channels. Furthermore, the computing device 22may be capable of monitoring multiple parameters and may determine theeffect of the optical test signal on the other channels based on acombination of parameters.

As shown in FIGS. 2, 3, and 4, the system 5 may include components alongthe fiber pathway 24 in addition to the transmitter 26, the first VOA20, and the receiver 28. For example, equipment foroptical-to-electrical-to-optical (o/e/o) translation 32 may be includedfor processing signal traffic information. In FIGS. 2, 3, and 4, forexample, two o/e/o nodes are included, each node having three o/e/ounits 32, or one pair of o/e/o units 32 for each live channel of thefiber pathway 24. Thus, in the figures, five of eight channels are darkand may be tested. The system 5 may also include a multiplexer 34 and ademultiplexer 36 between the o/e/o units 32. As previously discussed,the multiplexer 34 may be configured to multiplex, or join, signals ontochannels of the fiber pathway 24. The multiplexer 34 may be disposed atthe first end of the fiber pathway 24, for example downstream of theo/e/o units 32. The demultiplexer 36 may be configured to demultiplex,or split apart, signals from channels of the fiber pathway 24, such thateach signal would continue propagation along the fiber pathway 24 on aseparate optical fiber (not shown). Thus the demultiplexer 36 may bedisposed between an end of the fiber pathway 24 and the receiver 28, forexample as shown in FIG. 2.

Between the multiplexer 34 and the demultiplexer 36, additionalcomponents may be included to maintain the quality of the signals andotherwise manage signal traffic along the fiber pathway 24. For example,a number of optical amplifiers 38 may be included to insure that thestrength of signals propagated from one end of the fiber pathway 24 toanother end is at an appropriate level for downstream processing. Inaddition, one or more optical add/drop modules 40 may be included foradding and/or dropping certain channels along the fiber pathway 24.

In some embodiments, a second VOA 42 may be disposed at another locationof the fiber pathway 24. The second VOA 42 may be configured tocommunicate with the computing device 22 and to adjust attenuation ofthe optical test signal from an initial, greater attenuation to asubsequent, lesser attenuation. For example, in FIG. 2, the second VOA42 is disposed upstream of the receiver 28 and is configured toattenuate the optical test signal to reduce interference of thereflected test signal with signal traffic on other channels. Forexample, part of the optical test signal may be reflected by thereceiver 28 back toward the demultiplexer 36, causing interference withother channels. By adjusting the attenuation of the second VOA 42, forexample as directed by the computing device 22, the second VOA 42 canprovide additional protection for the live signal traffic.

Referring to FIG. 3, the system 5 may also include first and secondoptical switches 44, 46 configured to select a channel of the fiberpathway 24 for propagation of the optical test signal. For example, thefirst optical switch may be disposed between the first VOA 20 and themultiplexer 34, and the second optical switch 46 may be disposed betweenthe second VOA 42 and the demultiplexer 36, as shown in FIG. 3. Thefirst and second optical switches 44, 46 may be configured tocommunicate with the computing device 22 such that the computing device22 may direct the optical switches 44, 46 to select a channel (i.e., adark channel) of the fiber pathway 24 over which the optical test signalmay be propagated. In this way, multiple channel tests may be supported.

Other components of the fiber pathway 24 may vary depending on themeasurements or tests to be performed on the system. For example, thespectral response of the fiber pathway 24 may be measured by atransmitter 26 including a broadband source. The broadband source may beconfigured to propagate optical test signals having a respectivewavelength along the fiber pathway 24. The receiver 28 may include anoptical spectrum analyzer configured to analyze a wavelength spectrum ofthe optical test signal. In this way, the optical signal-to-noise ratio,pass band shape, and other characteristics of the fiber may bedetermined. Similarly, the receiver may include a polarization modedispersion analyzer configured to measure a polarization mode dispersionof the optical test signal.

In another example, the system 5 may include a tunable dispersioncompensator 48 disposed between an end of the fiber pathway 24 and thereceiver 28. For example, in FIG. 4, the tunable dispersion compensator48 is disposed between the demultiplexer 36 and the receiver 28. Thetunable dispersion compensator 48 is configured to adjust the dispersioncompensation function of the optical test signal, for example tocompensate for the chromatic dispersion of signals propagating along thefiber pathway 24 as a result of fiber imperfections.

In other embodiments, a method for characterizing optical fiberscarrying signal traffic is provided. An optical test signal isselectably attenuated prior to propagation along a channel of a fiberpathway. The optical test signal is then propagated along the channel ofthe fiber pathway, and the attenuation of the optical test signal isadjusted from an initial, greater attenuation to a subsequent, lesserattenuation while at least one other channel of the fiber pathway ismonitored and effects upon the other channel(s) from the optical testsignal are identified. See FIG. 5, blocks 100-110.

For example, as previously described, the attenuation of the opticaltest signal may initially be set at a very high value, such that whenthe optical test signal is propagated substantially all of the testsignal is attenuated. Attenuation of the optical test signal may then begradually decreased, allowing more of the optical test signal to bepropagated along the fiber pathway.

The attenuation of the optical test signal may continue to be adjusteduntil one of several situations arises, indicating that a maximumoptical test signal strength has been achieved. For example, the effectsupon the other channel(s) being monitored may result in a predefinedalarm condition, as discussed above, indicating that the optical testsignal has adversely affected one or more other channels. Block 108. Inthis situation, adjusting the attenuation of the optical test signal mayinclude stopping the decrease of attenuation of the optical test signalwhen the effects upon the other channel(s) results in the predefinedalarm condition, and then increasing the attenuation until thepredefined alarm condition is cleared. Block 112. In this way, a maximumsignal strength at which no alarm condition is triggered may beachieved.

Even if the optical test signal does not adversely affect any otherchannel, a predefined optical test signal strength may be achieved, suchas the highest test signal strength that can be propagated by thetransmitter or the test signal strength required to perform one or moretests or measurements on the system, as previously described. Block 110.In this case, a further decrease in attenuation of the optical testsignal strength, as shown in block 114, may not be possible ornecessary.

In determining whether the optical test signal strength has adverselyaffected any other channel, a bit error rate of the other channel(s) maybe monitored, and attenuation of the optical test signal may be adjustedsuch that the signal strength results in a bit error rate of the otherchannel(s) that is below a predetermined level. For example, attenuationof the optical test signal may continue to be decreased until the biterror rate reaches a certain value (e.g., a maximum acceptable bit errorrate), at which point the attenuation may be stopped. In this way, themaximum optical test signal strength at which the resulting bit errorrate of the other channel(s) is acceptable may be achieved.

In some embodiments, a channel of the fiber pathway may be selectedprior to the attenuation and propagation of the optical test signal, asshown in block 115. This may facilitate multiple channel tests accordingto this method. In addition, a wavelength of the optical test signal maybe selected, block 120, for example to support measurements of theoptical signal-to-noise ratio, pass band shape, and other fibercharacteristics via an analysis of the wavelength spectrum of theoptical test signal, block 130. Likewise, a polarization mode dispersionof the optical test signal may be measured, as previously discussed.Block 130.

Furthermore, attenuation of the optical test signal at another locationalong the fiber pathway may be adjusted from an initial, greaterattenuation to a subsequent, lesser attenuation. See blocks 100, 112,114. As previously discussed, attenuation of the optical test signal atanother location may reduce or safeguard against the interference of thetest signal with the live signal traffic on other channels of theoptical fiber. For example, attenuation of the optical test signalupstream of a receiver at an end of the fiber pathway may reduce theinterference caused by a portion of the optical test signal that may bereflected by the receiver.

In the preceding specification, various embodiments have been described.It will, however, be evident that various modifications and changes maybe made thereunto without departing from the broader spirit and scope ofthe invention as set forth in the claims that follow. The specificationand drawings are accordingly to be regarded in an illustrative ratherthan restrictive sense.

1. A system, comprising: an optical fiber pathway; a transmitterdisposed at a first end of the optical fiber pathway and configured toprovide an optical test signal for propagation along the optical fiberpathway; a receiver disposed downstream from the transmitter along theoptical fiber pathway and configured to receive the optical test signalthat has been propagated along the optical fiber pathway; a variableoptical attenuator disposed between the transmitter and receiver on theoptical fiber pathway and configured to attenuate a strength of theoptical test signal; and a computing device communicatively coupled tothe transmitter, receiver, and variable optical attenuator andconfigured to monitor at least one channel of the optical fiber pathwayother than a channel by which the optical test signal is propagated,identify effects of the optical test signal on the at least one channel,and direct adjustment of an attenuation level imposed on the opticaltest signal by the variable optical attenuator.
 2. The system of claim1, wherein the variable optical attenuator is disposed proximate to thetransmitter.
 3. The system of claim 1, further comprising:optical-to-electrical-to-optical translation components disposed betweenthe transmitter and receiver along the optical fiber pathway andconfigured to process signal traffic information.
 4. The system of claim1, further comprising: a multiplexer disposed between the transmitterand the receiver along the optical fiber pathway and configured to joinsignals onto channels of the optical fiber pathway; and a demultiplexerdisposed downstream from the multiplexer between the transmitter andreceiver and configured to split apart signals from channels of theoptical fiber pathway.
 5. The system of claim 1, further comprising: atleas one optical amplifier disposed between the transmitter and receiveralong the optical fiber pathway and configured to amplify opticalsignals propagated along the optical fiber pathway.
 6. The system ofclaim 1, further comprising: at least one optical add/drop moduledisposed between the transmitter and receiver along the optical fiberpathway and configured to add or drop channels along the optical fiberpathway.
 7. The system of claim 1, further comprising: a second variableoptical attenuator disposed between the transmitter and receiver alongthe optical fiber pathway in proximity to the receiver andcommunicatively coupled to the computing device and configured toattenuate the optical test signal to reduce interference between areflection of the optical test signal caused by the receiver and othersignal traffic on other channels of the optical fiber pathway.
 8. Thesystem of claim 1, further comprising: at least one optical switchdisposed between the transmitter and the receiver along the opticalfiber pathway and configured to select a channel of the optical fiberpathway by which to propagate the optical test signal.
 9. The system ofclaim 1, wherein the receiver includes an optical spectrum analyzerconfigured to analyze a wavelength spectrum of the optical test signal.10. The system of claim 1, wherein the receiver includes a polarizationmode dispersion analyzer configured to measure a polarization modedispersion of the optical test signal.
 11. The system of claim 1,further comprising: a tunable dispersion compensator disposed betweenthe transmitter and receiver along the optical fiber pathway andconfigured to adjust a dispersion compensation function associated withthe optical test signal.
 12. A method, comprising: attenuating, at afirst attenuation level, an optical test signal propagating in anoptical fiber pathway by a variable optical attenuator; monitoring atleast one channel of the optical fiber pathway other than a channel bywhich the optical test signal propagates; and adjusting attenuation ofthe optical test signal by the variable optical attenuator based on themonitoring of the at least one channel.
 13. The method of claim 12,wherein adjusting attenuation of the optical test signal reducesattenuation of the optical test signal.
 14. The method of claim 12,wherein adjusting attenuation of the optical test signal proceeds untila predefined optical test signal strength is achieved.
 15. The method ofclaim 12, further comprising: detecting a predefined alarm condition inthe at least one channel of the optical fiber pathway based on themonitoring; and wherein adjusting attenuation of the optical test signalproceeds until the predefined alarm condition is detected.
 16. Themethod of claim 15, wherein the predefined alarm condition comprisesdeclination of a signal-to-noise ratio below a threshold level.
 17. Themethod of claim 15, wherein the predefined alarm condition comprisesincrease of a bit error rate above a threshold level.
 18. The method ofclaim 15, further comprising: selecting a channel of the optical fiberpathway for propagation of the optical test signal.
 19. The method ofclaim 15, further comprising: selecting a wavelength of the optical testsignal; and wherein monitoring the at least one channel of the opticalfiber pathway comprises measurement of at least one an opticalsignal-to-noise ratio or pass band shape of the at least one channel.20. The method of claim 15, further comprising: reducing interferencebetween a reflection of the optical test signal and signal traffic onother channels of the optical fiber pathway by attenuating the opticaltest signal by a second variable optical attenuator disposed inproximity to an optical receiver from which the reflection originates.